nixos/systemd-boot: always use profile_path() for system closure paths

[notgne2]

Description of changes

Since profile_path always looks at a sub-path of the system closure and calls os.path.realpath, this ensures the paths we use always face the exact content we want for systemd-boot, and not a link to them. This solves various issues with https://github.com/serokell/deploy-rs and anything else that may put certain paths behind symlinks.

For when we don't want a specific sub-path, we instead look for the parent of a specific sub-path, by still using profile_path() but wrapping it with os.path.dirname()

Fully resolves serokell/deploy-rs#31 (currently partially hacked around by nuking default out of the file)

Likely resolves serokell/deploy-rs#185 as long as using systemd-boot (others haven't been yet been tested).

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[tomberek]

Initial testing looks good.

[RaitoBezarius]

Apologies for the delay in the review, I wanted to do this when my mental load was low. :)
This seems a clear improvement and I do not see why it would produce any regression, at least, it should introduce new bugs, old ones can still lurk into that code.

@nikstur We should check what we do in lzbt regarding this kind of things.

[RaitoBezarius]

@notgne2 Can you fix the conflicts please? I think I would be eager to merge this as it has waited long enough.

[RaitoBezarius]

@ofborg test systemd-boot